1. Field of the Invention
The present invention generally relates to an apparatus for treating a sheet-like structure, for example, a plastics film or fabrics of the like, with plasma to improve the chemical, physical, mechanical, optical and/or electrical properties, and/or the surface structure of the sheet-like structure. More specifically, the present invention relates to a plasma treating apparatus comprising a vacuum vessel filled with a plasma and having a rotatably supported drum-shaped discharge electrode assembly over which the sheet-like structure is passed for the plasma treatment at low temperature.
2. Description of the Prior Art
Recently, the plasma treatment has attracted the attention of those skilled in the art as an effective method for treating a sheet-like structure, for example, a plastics film or fabrics with plasma to improve the chemical, physical, mechanical, optical and/or electrical properties, and/or the surface structure of the sheet-like structure. More specifically, it is well described in the Japanese Laid-open Patent Publication No. 57-18737, published in 1982, and the Japanese Laid-open Patent Publication No. 60-149441, published in 1985, that by using the plasma treatment, the weldability, the frictional property, the feeling or handling property, the luster, and/or the color fixing property of the sheet-like structure can be improved so as to avoid a build-up of an electrostatic charge, to facilitate the surface hardening or roughening, to avoid the occurrence of blocking or to permit the dyed color to be thickened.
The known plasma treatment apparatus for treating the sheet-like structure with plasma as disclosed in any one of the above mentioned publications makes use of, as is the case with the predecessor of such apparatus, a generally drum-shaped discharge electrode assembly fixedly mounted on a rotary shaft extending through a vacuum vessel and a generally rod-shaped discharge electrode assembly disposed within the vacuum vessel in a face-to-face relationship with the drum-shaped discharge electrode assembly. According to this prior art plasma treating apparatus, the arrangement has been made to avoid any possible occurrence of a plasma discharge from both of the discharge electrode assemblies into the vacuum vessel by insulating the vacumm vessel from, for example, an electric circuit system with which both of the discharge electrode assemblies are connected, thereby minimizing the consumption of electric power.
It has, however, been found that, since the drum-shaped discharge electrode assembly is directly mounted on the rotary shaft to enable rotation of the discharge electrode assembly. The rotary shaft is also electrically charged during the operation of the apparatus. Because of this, a plasma discharge tends to occur from the rod-shaped discharge electrode assembly to the rotary shaft, resulting in the consumption of a relatively increased amount of electric power.
Also, the plasma treatment apparatus designed specifically for the plasma treatment of a sheet-like structure tends to be bulky in view of the fact that the sheet-like structure to be treated with plasma has a substantial width; and therefore, in order for the plasma treatment apparatus to be adopted in practice by industries, the plasma treatment apparatus must have a handling capacity corresponding to the relatively great amount of plant investment needed to implement this apparatus. In other words, in order for the handling capacity of the plasma treatment apparatus to be increased, the input, originally large, to the plasma treatment apparatus for creating the plasma-filled atmosphere must be further increased to increase the plasma density within the vacuum vessel. However, within a vacuum vessel filled with a plasma having a very low electric resistance, the increase of an input would result in local unevenness of the electrical distribution between the discharge electric circuit, disposed within but electrically insulated from the vacuum vessel, and the vacuum vessel; and therefore, insufficient electric insulation may occur at a corner area; for example, at an area between the rotary shaft extending through the vacuum vessel and a portion of the vacuum vessel adjacent the rotary shaft. This would allowing an electric current to flow through the vacuum vessel causing the inner wall of the vacuum vessel to become electrically overcharged (being luminous) resulting in an instantaneous abnormal spark discharge. Accordingly, the plasma discharge occurring within the vacuum vessel becomes so unstable that the plasma treatment apparatus can no longer be continuously operated.